This invention relates to a ceramic capacitor having a low temperature coefficient of capacitance and being comprised of a lead lanthanum zirconate titanate (PLZT) phase and a small intergranular cadmium- and/or zincborate phase having gettered lead that escaped from the PLZT phase during sintering.
The use of a glass sintering aid in PLZT dielectric ceramics is disclosed in each of my patents U.S. Pat. No. 4,027,209 issued May 31, 1977, U.S. Pat. No. 4,135,224 issued Jan. 16, 1979, U.S. Pat. No. 4,219,866 issued Aug. 26, 1980, and U.S. Pat. No. 4,324,750 issued Apr. 13, 1982 all of which are assigned to the same assignee as is the present invention. In the last of these it is noted that small amounts of the lead are volatilized and escape from the PLZT as lead oxide PbO during calcining and sintering, and that precipitation of semiconducting PbO at the grain boundaries seriously degrades capacitor performance.
The addition of glass sintering aids noted in U.S. Pat. No. 4,027,209 reduced this source of degradation but used alone it was far from an adequate solution to that problem as seen in U.S. Pat. No. 4,324,750 example 6. There the PLZT with 1.0 weight percent of a cadmium zinc borate silicate exhibited an unacceptable life test result. A post-sinter annealing step in an open atmosphere at about 950.degree. C. for about an hour was executed to drive out the unwanted free lead oxide. That procedure produced a good life test result as in examples 7 and 8 of the later mentioned patent.
The annealing step is a solution to the free PbO problem but is accompanied by the disadvantage that the optimum anneal time and temperature must be determined for each formulation and is further a function of the surrounding PbO pressure at sintering and the quantity of ceramic being sintered in each closed crucible. In short, the anneal step is difficult to control.
It is therefore an object of this invention to provide a high quality PLZT dielectric ceramic formulation that does not require a post-sintering anneal.